1. Field of the Invention
The present invention relates to a NOx-decomposing electrode having a certain or high NOx-decomposing/reducing ability for decomposing a NOx to produce oxygen, and a NOx sensor for measuring a NOx in exhaust gas from vehicles, atmospheric air, etc.
2. Description of the Related Art
A method for measuring a NOx in a gas to be measured such as a combustion gas, which comprises using a sensor obtained by forming a NOx-decomposing electrode on an oxygen ion-conductive solid electrolyte such as zirconia, thereby measuring an electromotive force generated on the NOx-decomposing electrode, has been known (see Japanese Laid-Open Patent Publication No. 11-183434, etc.) The NOx-decomposing electrode is a cermet electrode composed of a Pt—Rh alloy and a ceramic component.
A NOx-decomposing electrode, which has a multilayer structure of a plurality of cermet electrode layers containing a Pt—Rh alloy and a ceramic component, the cermet electrode layers being different in the ratio between the Pt—Rh alloy and the ceramic component, has been disclosed (see Japanese Laid-Open Patent Publication Nos. 2003-322634 and 2003-322636, etc.) In this NOx-decomposing electrode, the ceramic component is preferably a partially or fully stabilized ZrO2, and the stabilizer may be Y2O3, MgO, CaO, CeO2, etc. It is preferable to use Y2O3 as the stabilizer in view of burning at a low temperature.
Further, an electrode-protecting layer formed by screen printing for covering a NOx-decomposing electrode, which is a porous ceramic layer having a substantially trapezoidal cross section, has been disclosed (see Japanese Laid-Open Patent Publication No. 2006-284223, etc.) The electrode-protecting layer has a flat upper base portion extending in the horizontal direction and has tapered portions at the both ends of the upper base portion. The height of each tapered portion decreases gradually from the upper base portion toward the end of the NOx-decomposing electrode.
NOx-decomposing electrodes are used at a high temperature of 700° C. to 800° C. to measure a NOx, and thereby are continually expanded and shrunk repeatedly. As a result of the repeated expansion and shrinkage, an electrode-protecting layer on the NOx-decomposing electrode is cracked or broken, or the NOx-decomposing electrode is peeled off from a solid electrolyte, in some cases.
In these cases, the whole or part of the NOx-decomposing electrode cannot function normally, thereby the impedance of a pumping cell in a NOx sensor is increased.